A New Approach to Grain Boundary Plane Engineering (Hyun Uk Hong, Changwon National Univ.)

Hyun Uk Hong
2014-05-30 pm 04:00 / EB1, E207

A New Approach to Grain Boundary Plane Engineering
by Serrated Transition

Hyun Uk Hong, Ph.D.
Department of Materials Science and Engineering
Changwon National University


Abstract

As the primary creep damage and weld heat-affected zone (HAZ) microfissuring is an intergranular phenomenon of Ni-base superalloys at elevated temperatures, the great concerns could be given to the character of grain boundaries (GBs). Recently, the present authors have found that GB serration through the special heat treatment designed proprietarily by the present authors, leads to a change in GB character as special boundary. The present work was initiated to study the possibility of GB serration as a viable means of improving the resistance of Ni-base superalloys to intergranular degradation. The formation of serrated GBs and its subsequent effect on creep resistance in wrought Ni-base superalloys Alloy 263, 617 and Haynes 230 have been studied. The present study presents a new finding that the GB serration occurs without the presence of M23C6 or γ' phase. The crystallographic analysis revealed that the GBs tend to serrate to have specific segments approaching the {111} low-index plane at a boundary so that interfacial free energy of GB can be decreased, which may be responsible for the driving force of GB serration. The formation of serrated GBs with lower energy leads to a change in the carbide characteristics. Electron energy-loss spectroscopy (EELS) studies suggest that serration is triggered by the discontinuous segregation of C and Cr atoms at GBs for the purpose of relieving the excessive elastic strain energy. The role of this lattice distortional energy due to the atomic size difference is confirmed on the serration behaviors through binary Ni-xZr model alloy system. The serrated GBs with stable planar M23C6 carbides enhanced creep lives due to their high resistance to GB migration accompanied by cavitational cracking. The factors affecting creep properties are discussed in terms of GB strengthening.